1. Field of the Invention
The present invention relates to a charge transfer device (CTD) for use in solid-state image pickup devices, delay lines, and the like.
2. Description of the Prior Art
An example of currently available charge transfer devices (CTD) for use in a solid-state image pickup device is shown in FIG. 3. This charge transfer device has a transfer section 101, a pickup section 102, and an amplification section 103, all provided on a substrate 100. Transfer gates G1-G4 at the stages of the transfer section 101 are driven by two-phase clocks .phi..sub.1, .phi..sub.2 applied thereto alternately from external and transfer a signal charge along a linear transfer channel X--X' toward the pickup section 102. In the figure, the hatched portions represent barrier regions which function to determine the direction in which the signal charge is transferred. The signal charge deriving from the transfer section 101 passes under an output gate OG, to which a DC voltage Vog is being applied, and enters a floating diode FD, which constitutes the pickup section 102. Then a potential change of the floating diode FD corresponding to an amount of the signal charge is transferred to the amplification section 103. The amplification section 103, made up of MOS transistors Tr.sub.1, Tr.sub.2, Tr.sub.3, and Tr.sub.4 provided between the power supply OD and ground, amplifies the voltage signal coming from the floating diode FD and provides its output signal OS to a connecting point of the MOS transistors Tr.sub.2 and Tr.sub.3. It is noted that the floating diode FD is periodically restored to a specified reset potential V.sub.RD when a clock .phi.r (to be applied synchronously with the two-phase clocks .phi..sub.1, .phi..sub.2) is applied to a reset gate RG from external of the device.
As is known, to enhance the charge-to-voltage conversion factor of the floating diode FD, it is desirable to reduce the area of the floating diode FD to thereby reduce its electrostatic capacity. In the conventional device, the width of the transfer channel X--X' is narrowed below the output gate OG connecting the transfer section 101 and the pickup section 102, and the area of the floating diode FD is reduced accordingly.
However, the conventional charge transfer device has the following drawback. That is, since the width of the linear transfer channel X--X' is merely narrowed, there arises a potential barrier 20 slanted in such a direction as will obstruct the transfer, to the width-narrowed point (i.e., below the output gate OG), as shown in FIG. 2 (a) during the charge transfer. This potential barrier 20 causes some transfer fault and, especially when the charge transfer device is used in a solid-state image pickup device, adversely affects its characteristic for low illuminance.